#coding:UTF-8
from tkinter import *

root=Tk()

w=Canvas(root,width=400,height=400,background='white')
w.pack()

def paint(event):
    x1,y1=(event.x-1),(event.y-1)
    x2,y2=(event.x+1),(event.y+1)
    w.create_oval(x1,y1,x2,y2,fill="black")



w.bind('<B1-Motion>',paint)

mainloop()